1. Field of the Invention
An aspect of the present invention relates to a method and apparatus of reproducing data recorded on a super-resolution information storage medium, and more particularly, to a method and apparatus of reproducing data recorded on a super-resolution information storage medium which can improve characteristics of a reproduced signal by removing inter-symbolic interference (ISI) from the super-resolution information storage medium.
2. Description of the Related Art
An optical recording medium is used as an information storage medium of an optical pickup device for recording and/or reproducing information in a non-contact type. With the progress of the industrial development, information recording media having greater recording density are highly in demanded. Accordingly, development of optical recording media capable of reproducing recording marks having a spot diameter less than a laser beam spot using a super resolution phenomenon is under way.
In general, when a wavelength of light for reproducing data recorded on a recording medium is λ, and a numerical aperture of an objective lens is NA, the limit of reproducible resolution becomes λ/4NA. In other words, since light emitted from a light source is unable to distinguish recording marks having a diameter smaller than λ/4NA from others, it is very often that such data is not reproducible.
However, a recording mark exceeding such a resolving power limit may be reproduced, which is referred to as a super resolution phenomenon. Nowadays, investigation of causes of the super resolution phenomenon and research and development into the super resolution phenomenon are under way. Since super resolution enables reproduction of a recording mark exceeding a resolving power limit, a super resolution information storage medium can markedly realize demands for high density and large storage capacity.
Requirements for commercial use of super-resolution information storage media are that the information storage media satisfy basic recording and reproducing features as storage media. In particular, super resolution information storage media utilize recording beams and reproduction beams having relatively high power compared to conventional information storage media. Furthermore, super resolution information storage media have major issues with reproduction signal characteristics, such as carrier-to-noise ratio (CNR), jitter or RF signal, and with the realization of stable reproduction signals. In order to place super resolution information storage medium into practice, it is a prerequisite for the super resolution information storage medium to satisfy reproduction signal characteristics.
An area of a reproduction beam spot on a super resolution information recording medium where super resolution phenomenon occurs will now be described with reference to FIG. 1.
As shown in FIG. 1, marks 110 are recorded on a track 100 of a super resolution information storage medium, and a change in the temperature distribution or optical property occurs within a beam spot 120 landing on a super resolution layer due to a difference in local light intensity. Thus, marks 110 beyond a resolving power limit may also be reproduced. In other words, a change in the temperature distribution or optical characteristic occurs at a partial region of the beam spot 120, and no changes occur at a peripheral area 140 of the partial region. The partial region where such a change occurs, which will be referred to as a super resolution area 130 hereinafter, may be a central portion, as shown in FIG. 1. Such areas where a change in optical characteristics occurs may be consecutive or alternate.
Actually, there are many reports indicating that a CNR large enough to be applied to a practical medium was obtained from marks of the same lengths that are smaller than a resolving power by super resolution reproducing operations using various super-resolution materials. However, actual optical recording is executed not by recording marks of the same lengths at regular intervals but by recording marks of the same lengths at irregular intervals (i.e., a mark position detecting method) or by recording marks of different lengths at irregular intervals (i.e., a mark length detection method). Particularly, in CDs or DVDs, marks of various lengths ranging between 3T and 11T (where T denotes a clock frequency) are complexly recorded. However, none of the above-described super resolution techniques has yet succeeded in reproducing such a complex signal, because signals reflected from an optical recording medium contain not only signals reflected from the area of the beam spot where optical characteristics change but also signals reflected from a peripheral area of the area where optical characteristics change. If there is no signal from the peripheral area, the size of an effective beam spot is substantially reduced, so that a complex signal can be reproduced. However, in the above-described super resolution techniques, a difference between the area where optical characteristics change and the peripheral area is used, and since the difference is small, signals reflected from the peripheral area serve as an obstacle to the spot size reduction. This results in ISI (Inter Symbolic Interference) that occurs when a series of marks are reproduced, so that a complex signal cannot be reproduced with a high resolution.
FIG. 2A illustrates a recording pattern of marks recorded on an information storage medium, and FIG. 2B illustrates an RF signal corresponding to reproduced marks of the recording pattern shown in FIG. 2A. When a wavelength of a laser beam is 405 nm, an NA thereof is 0.85, and a resolving power thereof is approximately 75 nm, the recording pattern is based on a combination of a mark of approximately 75 nm, which is smaller than a resolving power, a mark of approximately 300 nm, which is greater than the resolving power, and a space between the two marks. In the reproduction signal shown in FIG. 2B, when a 300 nm long mark or space is present around a beam spot, a 75 nm long mark is affected by the 300 nm long mark and space so that it is not possible to clearly detect the 75 nm long mark. Areas having 75 nm long marks are indicated by A, B, C, D, E, and F. Referring to FIGS. 2A and 2B, levels of the reproduction signal for the areas A, B, C, D, E, and F are different according to the numbers of 75 nm long marks and spaces. Further, each of the levels of the reproduction signal for the areas A, B, C, D, E, and F is not constant but variable depending on the surrounding conditions of the 75 nm long mark.
The above-stated problems are caused due to ISI of signals from the peripheral area 140 of the beam spot.